Micromachined silicon structures are used frequently as sensors and actuators, and for signal processing. For example, the fingers of a movable mass may be positioned between stationary plates to form a differential capacitor. The use of such a device as an accelerometer and a method for fabricating such an accelerometer are described in U.S. Pat. No. 5,345,824, entitled "Monolithic Accelerometer", and U.S. Pat. No. 5,326,726, entitled "Method for Fabricating Monolithic Chip Containing Integrated Circuitry and Suspended Microstructure", which are incorporated herein by reference.
As these micromachined devices become more complex, it becomes desirable to use more conductive layers. Instead of just a single polysilicon layer used for a movable micromachined structure, it is desirable to use an additional polysilicon layer. This additional layer might be above the movable structure, to contain the movable layer or to serve as an electrode for a differential capacitor. This additional layer might be below the movable structure, to provide an extra layer of interconnects with closer spacing than would be possible with diffused regions.
It is also desirable to employ a conductive plane (often referred to as a "ground plane") underneath the movable structure. For example, placing a conductive plane under the structure (the conductive plane being separated from the substrate by a dielectric) and keeping the plane at approximately the same electric potential as the structure minimizes the electrostatic attractive force that would tend to pull the structure down to the substrate.
However, if the bottom polysilicon layer is used to provide an extra layer of interconnects, the gaps in the layer detract from the effectiveness of that layer as a ground plane. The potential of the substrate beneath the gaps is unspecified and the dielectric layer between the substrate and the conductive plane may have charges on it that can cause spurious forces on the microstructure and measurement errors.
The present invention overcomes this and other problems as will be shown in the remainder of the specification referring to the attached drawings.